Acid mine drainage (AMD), or acid rock drainage (ARD), refers to the outflow of acidic water from (usually abandoned) metal mines or coal mines. However, other areas where the earth has been disturbed (e.g. construction sites, subdivisions, transportation corridors, etc.) may also contribute acid rock drainage to the environment. In many localities the liquid that drains from coal stocks, coal handling facilities, coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. Acid rock drainage occurs naturally within some environments as part of the rock weathering process but is exacerbated by large-scale earth disturbances characteristic of mining and other large construction activities, usually within rocks containing an abundance of sulfide minerals.
The same type of chemical reactions and processes may occur through the disturbance of acid sulfate soils formed under coastal or estuarine conditions after the last major sea level rise, and constitute a similar environmental hazard.
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Sub-surface mining often progresses below the water table, so water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping ceases, and water floods the mine. This introduction of water is the initial step in most acid rock drainage situations. Tailings piles or ponds may also be a source of acid rock drainage.
After being exposed to air and water, oxidation of metal sulfides (often pyrite, which is iron-sulfide) within the surrounding rock and overburden generates acidity. Colonies of bacteria and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen supplies usually keep their numbers low. Special extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. In particular, Acidithiobacillus ferrooxidans is a key contributor to pyrite oxidation.[1]
Metal mines may generate highly acidic discharges where the ore is a sulfide mineral or is associated with pyrite. In these cases the predominant metal ion may not be iron but rather zinc, copper, or nickel. The most commonly mined ore of copper, chalcopyrite, is itself a copper-iron-sulfide and occurs with a range of other sulfides. Thus, copper mines are often major culprits of acid mine drainage.
For further information, see Acidophiles in acid mine drainage
The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options.[2]
Although a host of chemical processes contribute to acid mine drainage, pyrite oxidation is by far the greatest contributor. A general equation for this process is:
The oxidation of the sulfide to sulfate solubilizes the ferrous iron (iron(II)), which is subsequently oxidized to ferric iron (iron(III)):
Either of these reactions can occur spontaneously or can be catalyzed by microorganisms that derive energy from the oxidation reaction. The ferric irons produced can also oxidize additional pyrite and oxidize into ferrous ions:
The net effect of these reactions is to release H+, which lowers the pH and maintains the solubility of the ferric ion.
Traditionally, the character of acid mine drainage is determined by its acidity (mg/L), which is measured by titrating AMD with sodium hydroxide solution from the AMD initial pH till pH 8.3. Then calculate the moles of NaOH that consumed by one liter of AMD, and transfer the mole number into the weight of CaCO3. It is the value of acidity (mg/L) of AMD. Hence, the direct meaning of acidity is: weight of CaCO3 needed to neutralize the pH of 1 liter AMD.
However, acidity can not best represent AMD’s characters. Some AMDs have same acidity values, even same pH value, but of different properties. Because the AMD acidity includes two components: hydrogen ions and dissolved metal ions. This can be seen clearly from the AMD acidity titration curves.
The AMD acidity titration curve is shaped like a staircase. Vertical part shows the process OH- ions neutralizing H+ ions, which increases the pH of water. Horizontal part indicates OH- ions precipitate metal ions into metal hydroxides, which will act as a buffer, using hydroxide from the titrant, keeping the pH constant for a brief time until a specific metal has completely precipitated.
Most metal hydroxides (except Na and K) are insoluble in water and have specific solubility products. When pH reaches certain level the metal ions will precipitate and be eliminated from the water. This forms the stair steps of the titration curve.
In different AMD the metal concentration may range from 500mg/L to 0.1mg/L. If assuming the highest concentration allowed for each metal is 0.1mmole/L, use metals’ precipitation products, we can calculate the criteria pH for each metal in water. Higher than the pH criteria the metal concentration is lower than 0.1mmole/L.
The following table shows the experimental and theoretical pH criteria of metals in AMD
| Metal | Fe+3 | Al+3 | Cu+2/Mn | Zn+2/Ni+2 | Fe+2 |
|---|---|---|---|---|---|
| pH1 | <3.2 | 3-4.5 | 5-6.5 | 6.5-8 | 5.5-6.5 |
| pH2 | 2.93 | 4.43 | 6.60/9.33 | 7.83/8.22 | 8.95 |
pH1 values are experiment data pH2 values are calculated from metal hydroxide solubility products, assuming metal concentration is 0.1mmole/L
In real situation the following factors can affect the metal precipitation process, therefore in the experiment pH of precipitation spans around 1-1.5:
The major advantage of titration curve method is: It is simple, convenient and low cost. It can show water problem quickly and clearly. It helps to explain the AMD forming procedure and helps to find the treat method.
In some acid mine drainage systems temperatures reach 117 degrees Fahrenheit (47 °C), and the pH can be as low as -3.6.[3]
Acid mine drainage causing organisms can thrive in waters with pH very close to zero. Negative pH occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions.
About half of the coal mine discharges in Pennsylvania have pH under 5 standard units (source needed). However, a significant portion of mine drainage in both the bituminous and anthracite regions of Pennsylvania is alkaline, because limestone in the overburden neutralizes acid before the drainage emanates.
Acid mine drainage has recently been a hindrance to the completion of the construction of Interstate 99 near State College, Pennsylvania, but this acid rock drainage didn't come from a mine: pyritic rock was unearthed during a road cut and then used as filler material in the I-99 construction. A similar situation developed at the Halifax airport in Canada. It is from these and similar experiences that the term acid rock drainage has emerged as being preferable to acid mine drainage, thereby emphasizing the general nature of the problem.
When the pH of acid mine drainage is raised past 3, either through contact with fresh water or neutralizing minerals, previously soluble Iron(III) ions precipitate as Iron(III) hydroxide, a yellow-orange solid colloquially known as yellow boy[4]. Other types of iron precipitates are possible, including iron oxides and oxyhydroxides. All these precipitates can discolor water and smother plant and animal life on the streambed, disrupting stream ecosystems (a specific offense under the Fisheries Act in Canada). The process also produces additional hydrogen ions, which can further decrease pH. In some cases, the concentrations of iron hydroxides in yellow boy are so high, the precipitate can be recovered for commercial use in pigments[5]
Many acid rock discharges also contain elevated levels of potentially toxic metals, especially nickel and copper with lower levels of a range of trace and semi-metal ions such as lead, arsenic, aluminium, and manganese. In the coal belt around the south Wales valleys in the UK highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome.
In the United Kingdom, many discharges from abandoned mines are exempt from regulatory control. In such cases the Environment Agency working with partners such as the Coal Authority have provided some innovative solutions, including constructed wetland solutions such as on the River Pelenna in the valley of the River Afan near Port Talbot and the constructed wetland next to the River Neath at Ynysarwed.
Although abandoned underground mines produce most of the acid mine drainage, some recently mined and reclaimed surface mines have produced ARD and have degraded local ground-water and surface-water resources. Acidic water produced at active mines must be neutralized to achieve pH 6-9 before discharge from a mine site to a stream is permitted.
In Canada, work to reduce the effects of acid mine drainage is concentrated under the Mine Environment Neutral Drainage (MEND) program. Total liability from acid rock drainage is estimated to be between $2 billion and $5 billion CAD.[6] Over a period of eight years, MEND claims to have reduced ARD liability by up to $400 million CAD, from an investment of $17.5 million CAD.[7]
By far, the most commonly used commercial process for treating acid mine drainage is lime precipitation in a high-density sludge (HDS) process. In this application, a slurry of lime is dispersed into a tank containing acid mine drainage and recycled sludge to increase water pH about ~9. At this pH, most toxic metals become insoluble and precipitate, aided by the presence of recycled sludge. Optionally, air may be introduced in this tank to oxidize iron and manganese and assist in their precipitation. The resulting slurry is directed to a sludge-settling vessel, such as a clarifier. In that vessel, clean water will overflow for release, whereas settled metal precipitates (sludge) will be recycled to the acid mine drainage treatment tank, with a sludge-wasting side stream. A number of variations of this process exist, as dictated by the chemistry of ARD, its volume, and other factors.[8] Generally, the products of the HDS process also contain gypsum and unreacted lime, which enhance both its settleability and resistance to re-acidification and metal mobilization.
Less complex variants of this process, such as simple lime neutralization, may involve no more than a lime silo, mixing tank and settling pond. These systems are far less costly to build, but are also less efficient (i.e., longer reaction times are required, and they produce a discharge with higher trace metal concentrations, if present). They would be suitable for relatively small flows or less complex acid mine drainage.[9]
A calcium silicate feedstock, made from processed steel slag, can also be used to neutralize active acidity in AMD systems by removing free hydrogen ions from the bulk solution, thereby increasing pH. As the silicate anion captures H+ ions (raising the pH), it forms monosilicic acid (H4SiO4), a neutral solute. Monosilicic acid remains in the bulk solution to play many roles in correcting the adverse effects of acidic conditions. In the bulk solution, the silicate anion is very active in neutralizing H+ cations in the soil solution.[10] While its mode-of-action is quite different from limestone, the ability of calcium silicate to neutralize acid solutions is equivalent to limestone as evidenced by its CCE value of 90-100% and its relative neutralizing value of 98%.[11]
In the presence of heavy metals, calcium silicate reacts in a different manner than limestone. As limestone raises the pH of the bulk solution and heavy metals are present, precipitation of the metal hydroxides (with extremely low solubilities) is normally accelerated and the potential of armoring of limestone particles increases significantly.[12] In the calcium silicate aggregate, as silicic acid species are absorbed onto the metal surface, the development of silica layers (mono- and bi-layers) lead to the formation of colloidal complexes with neutral or negative surface charges. These negatively charged colloids create an electrostatic repulsion with each other (as well as with the negatively charged calcium silicate granules) and the sequestered metal colloids are stabilized and remain in a dispersed state - effectively interrupting metal precipitation and reducing vulnerability of the material to armoring.[10]
Generally, limestone or other calcareous strata that could neutralize acid are lacking or deficient at sites that produce acidic rock drainage. Limestone chips may be introduced into sites to create a neutralizing effect. Where limestone has been used, such as at Cwm Rheidol in mid Wales, the positive impact has been much less than anticipated because of the creation of an insoluble calcium sulfate layer on the limestone chips, binding the material and preventing further neutralization.
Cation exchange processes have previously been investigated as a potential treatment for acid mine drainage. The principle is that an ion exchange resin can remove potentially toxic metals (anionic resins), or chlorides and sulfates (cationic resins) from mine water. Once the contaminants are adsorbed, the exchange sites on resins must be regenerated, which typically requires expensive reagents and generates a brine that is difficult to dispose. A South African company claims to have developed a patented ion-exchange process that treats mine effluents (and AMD) economically, but such claims remain unsubstantiated at present.
Constructed wetlands systems have been proposed during the 1980s to treat acid mine drainage generated by the abandoned coal mines in Eastern Appalachia.[13] Generally, the wetlands receive near-neutral water, after it has been neutralized by (typically) a limestone-based treatment process.[14] Metal precipitation occurs from their oxidation at near-neutral pH, complexation with organic matter, precipitation as carbonates or sulfides. The latter results from sediment-borne anaerobic bacteria capable of reverting sulfate ions into sulfide ions. These sulfide ions can then bind with heavy metal ions, precipitating heavy metals out of solution and effectively reversing the entire process.
The attractiveness of a constructed wetlands solution lies in its relative low cost. They are limited by the metal loads they can deal with (either from high flows or metal concentrations), though current practitioners have succeeded in developing constructed wetlands that treat high volumes (see description of Campbell Mine constructed wetland) and/or highly acidic water (with adequate pre-treatment). Typically, the effluent from constructed wetland receiving near-neutral water will be well-buffered at between 6.5-7.0 and can readily be discharged. Some of metal precipitates retained in sediments are unstable when exposed to oxygen (e.g., copper sulfide or elemental selenium), and it is very important that the wetland sediments remain largely or permanently submerged.
An example of an effective constructed wetland is on the Afon Pelena in the River Afan valley above Port Talbot where highly ferruginous discharges from the Whitworth mine have been successfully treated.
Most base metals in acidic solution precipitate in contact with free sulfide, e.g. from H2S or NaHS. Solid-liquid separation after reaction would produce a base metal-free effluent that can be discharged or further treated to reduce sulfate, and a metal sulfide concentrate with possible economic value.
As an alternative, several researchers have investigated the precipitation of metals using biogenic sulfide. In this process, Sulfate-reducing bacteria oxidize organic matter using sulfate, instead of oxygen. Their metabolic products include bicarbonate, which can neutralize water acidity, and hydrogen sulfide, which forms highly insoluble precipitates with many toxic metals. Although promising, this process has been slow in being adopted for a variety of technical reasons.[15]
With the advance of Large-scale sequencing strategies, genomes of microorganisms in the acid mine drainage community are directly sequenced from the environment. The nearly full genomic constructs allows new understanding of the community and able to reconstruct their metabolic pathways.[16] Our knowledge of Acidophiles in acid mine drainage remains rudimentary: we know of many more species associated with ARD than we can establish roles and functions.
This list includes both mines producing acid mine drainage and river systems significantly affected by such drainage. It is by no means complete, as worldwide, several thousands of such sites exist.